1. Field of the Invention
The present invention relates to an internal combustion engine and a motorcycle equipped with the internal combustion engine.
2. Description of the Related Art
Conventionally, in an internal combustion engine for motorcycles or the like, the cylinder head is provided with intake valves and exhaust valves for opening/closing a combustion chamber as well as a camshaft for driving these valves. The camshaft is linked to a crankshaft via a cam chain and is rotated as the crankshaft rotates. The cam chain is accommodated in a cam chain chamber, which is formed in the cylinder head and a cylinder body positioned below the cylinder head.
The tension on the cam chain changes depending on the rotational speed of the crankshaft. A structure for maintaining the tension on the cam chain at an appropriate level is known. In the known structure, a chain guide for guiding the cam chain and a chain tensioner for applying a force to the cam chain via the chain guide are disposed in the cam chain chamber. Oil is circulated in the cam chain chamber, and the oil scattered from the rotating cam chain is supplied to the chain tensioner. JP 2012-077655 A discloses a technique of supplying oil to a chain tensioner device by causing the oil spattered from a cam chain to collide against a wall provided in the cylinder head.
The chain tensioner may have a cylindrical case, a plunger sliding in the case, a spring for biasing a chain guide against a cam chain, and a ratchet that comes into contact with the plunger and inhibits the plunger from moving in a direction away from the chain guide. A problem with such a chain tensioner is that, when a sufficient amount of oil is not supplied to the chain tensioner, the chain tensioner does not work appropriately, and moreover, strange noise occurs at the contact surface between the ratchet and the plunger when in operation. In view of the problem, it is necessary to supply oil to the chain tensioner actively to smoothly operate the chain tensioner. However, if a passage for supplying oil directly to the chain tensioner is formed as a separate part from the cylinder head and the cylinder body, a complex structure and a higher cost result.